A touch screen offers a simple, intuitive interface to a computer or other data processing device. Rather than using a keyboard for data entry, a user can transfer information through a touch screen by touching an icon or by writing or drawing on a screen. Touch screens are used in a variety of information processing applications. Transparent touch screens, used over an information display such as a liquid crystal display (LCD) or cathode ray tube (CRT), are particularly useful for applications such as cellphones, personal data assistants (PDAs), and handheld or laptop computers.
Various methods have been used to determine touch location, including capacitive, resistive, acoustic and infrared techniques. Touch location may also be determined by sensing the force of the touch on the touch screen using force sensors coupled to a touch surface. Touch screens that operate by sensing touch force have several advantages over the other technologies mentioned above. Electrically based approaches, such as resistive and capacitive approaches, require a complex touch surface overlay that uses special materials and multiple layers to ensure good electrical properties across the screen while also maintaining good optical transmission through the screen. The overlay of a force-based touch screen, on the other hand, may be formed from a simple, single sheet of material. Further, force sensors do not rely on a lossy electrical connection to ground, as required by a capacitive touch screen, and can be operated by a finger touch, gloved hand, fingernail or other nonconductive touch instrument. Unlike surface acoustic wave technology, force sensors are relatively immune to accumulations of dirt, dust, or liquids on the touch surface. Finally, a force sensor is less likely to detect a close encounter as an actual touch, which may be a problem with infrared and capacitive touch screens.
Forces detected by touch screen force sensors reflect a variety of static and dynamic factors in addition to the touch force. These factors may be considered noise sources with respect to the touch signal. Noise may be introduced through the touch screen electronics, or it may be mechanical in nature. Electrical noise may be introduced, for example, in the sensor, amplifier, data conversion or signal processing stages. Mechanical noise may arise from various mechanical effects, such as vibration, flexure, movement, and the application of forces that are non-perpendicular to the touch screen. In addition, the touch screen force sensors may be affected by the weight of the touch surface and preloading forces applied to the force sensors during manufacture.